What is electric current?
It is very easy to see/hear/feel/etc. the effects of electric current in circuits that include things like lights/motors/speakers/etc. In addition, once you know how to do so safely, it is really easy to measure electric current with a multi-meter. Unfortunately, It is impossible for humans to detect current itself. Therefore, understanding the theory behind current takes longer to understand, and will be addressed later in this page.
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Quick list of important topics
- The unit for electrical current is the Ampere (often shortened to Amp). Symbol: A. 1A is larger than the current of most basic electronic circuits. So it helps a lot to be able to quickly convert amps into milliamps. 1A = 1,000mA. 0.001A – 1mA, and so on.
- 1A of current means that there is 1 coulomb of charge per second moving past any given point in a a series circuit. 1 coulomb is about 6×1018 negatively charged electrons. We now know that negatively charged electrons move from negative to positive, but early scientist thought that positive charges move from positive to negative. Most of the time, we still imagine positive charges moving from + to negative, calling it conventional current. Many universally accepted schematic symbols were made before electrons were discovered, and they have arrows pointing from positive to negative to help you get the polarity correct.
- Ohms law. Resistive components have a relationship between voltage and current that is really straightforward. In fact, it is a linear relationship. Ohms law for current is I = V/R (current is equal to volts divided by resistance).
Noticing the effects of circuit current:
To really understand circuits, you need to analyze the voltage and currents involved. Some components, such as LEDs and motors, make it really clear that there is some current going through them.
Properly connected (forward-biased) LEDs light up when a small amount of current passes through them. The more current through a forward biased LED, the brighter it gets. However, too much current will burn out an LED. Therefore, Resistance (usually provided by a resistor component) must almost always be used to limit current. If an LED is inserted backwards (reverse-biased) it will block a lot of voltage and not light up.
A motor turns faster when more current is flowing through it. Unfortunately, too much current will burn a motor out and maybe cause other damage due to it spinning too fast. Motors have resistance, so you can connect them to a power supply/voltage source that is within their rated voltage.
Therefore, current makes many component useful. However, too much current will cause damage.
Electrical current measurements with a multimeter:
Luckily, it is often very easy to measure the current of a series (single path) circuit. Most multi-meters have the ability to measure current.
Parallel (multi path) will have separate current paths that may combine somewhere. It is much better to use Ohms law and circuit analysis to calculate the current.
Electrical current theory
To begin with, current must be created. Doing so takes voltage from a power source such as a battery. Batteries have a positive and negative terminal. The voltage between a fresh 9 volt (usually written as 9V) alkaline battery’s terminals is about 9 volts. The terminals should be separated by an electrically insulative material (which includes air under normal conditions). Bridging that gap with an electrically conductive load however, will allow that voltage to push charges through the circuit that is formed.
- (conventional) Current – Imaginary positive charges flowing like a liquid from the positive side of a voltage source, through a load and back into the negative side of the power supply.
- Electron flow – Actual shuffling of electrons from the negative side of a power supply, through load and returning to the positive side of the power supply.
The flow of electrical charges through a circuit is usually referred to as current because of historical misunderstandings of electricity and the well established design of circuits. However, when there is a need or desire to be scientifically accurate, the moving charges are referred to as electron flow.
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